Endoglin, also known as, inter alia, CD105 or edg-1, is a type I homodimeric membrane glycoprotein which is expressed at high levels in proliferating vascular endothelial cells (Burrows et al., 1995, Clin. Cancer Res. 1:1623-1634). However, there is some expression of endoglin by the vascular endothelium of normal tissues (Burrows et al., Id; Wang et al., 1993, Int. J. Cancer 54:363-370). Human endoglin is known to specifically bind transforming growth factor-β (TGF-β), and the deduced amino acid sequence of endoglin has strong homology to β-glycan, a type of TGF-β receptor.
Several anti-endoglin antibodies, in particular anti-endoglin monoclonal antibodies (“mAb”), have been described. mAb SN6j is a monoclonal antibody generated from immunization of mice with glycoprotein mixtures of cell membranes of human leukemia cells (Haruta and Seon, 1986, Proc. Natl. Acad. Sci. USA 83:7898-7902). SN6 is a murine mAb that recognizes human endoglin. C-SN6j is a chimeric form of SN6j which has mouse variable regions and a human constant region. mAb 44G4 is an antibody generated from immunization of mice with whole cell suspensions of human pre-B leukemia cells (Gougos and Letarte, 1988, J. Immunol. 141:1925-1933; 1990, J. Biol. Chem. 265:8361-8364). 44G4 is also a murine mAb that recognizes human endoglin. mAb MJ7/18 is an antibody generated from immunization of rats with inflamed mouse skins (Ge and Butcher, 1994, Id). MJ7/18 is a mAb that recognizes murine endoglin. mAb Tec-11 is an antibody generated from immunization of mice with human umbilical vein endothelial cells (Burrows et al., 1995, Clin. Cancer Res. 1:1623-1634). Tec-11 is a murine mAb with reactivity restricted to human endoglin (Burrows et al., Clin. Cancer Res., 1995; 1(12): 1623-34).